1. Field of the Invention
The present invention relates to a method for determining a dead time TS in the response characteristic of an exhaust gas sensor in order to determine an exhaust gas state quantity in an exhaust gas duct of an internal combustion engine, the determination of the dead time TS taking place from a measured output signal of the exhaust gas sensor or a measured characteristic quantity derived therefrom, and from a comparison signal or comparison characteristic quantity derived therefrom. In addition, the present invention relates to a control unit for controlling an internal combustion engine and for determining a dead time TS in the response characteristic of an exhaust gas sensor, the control unit containing means for measuring an output signal of the exhaust gas sensor and/or for determining a characteristic quantity derived therefrom, and the control unit containing means for determining a comparison signal and/or for determining a comparison characteristic quantity derived therefrom.
2. Description of the Related Art
In the context of on-board diagnosis (OBD) for the operation of internal combustion engines, OBD-II regulations require that lambda sensors and other exhaust gas sensors be monitored not only with regard to their electrical functional capability, but also with regard to their response characteristic. A worsening of the sensor dynamic, which can become noticeable through an enlarged time constant or a longer dead time TS, must be recognized. Such time delays between a change in the composition of the exhaust gas and the recognition thereof must be monitored on board to see whether they are still permissible for user functions, i.e. for control, regulation, and monitoring functions that use the sensor signal. As a characteristic quantity for the dynamic characteristics of exhaust gas sensors, typically the dead time TS of a change in the mixture of the fuel-air mixture supplied to the internal combustion engine up to the associated signal edge of the exhaust gas sensor is used. The dead time TS is determined primarily by the gas runtime from the outlet of the internal combustion engine to the exhaust gas sensor, and changes for example when there is a manipulation of the location of installation of the sensor.
In order to set an air-fuel mixture supplied to the internal combustion engine, standardly lambda sensors are used as exhaust gas sensors, which determine the oxygen content of the exhaust gas. In diesel engines, for example lambda sensors in the form of broadband lambda sensors can be used. In addition, in connection with SCR (Selective Catalytic Reduction) catalytic converters for the conversion, using urea, of nitrogen oxides into carbon dioxide, nitrogen, and water, NOx sensors are provided. NOx sensors additionally supply an oxygen signal.
The dead time TS of the oxygen signal is monitored, using known methods, when there is a transition of the internal combustion engine from load to overrun. Here, the oxygen portion increases from a portion specified by the operating point of the internal combustion engine under load to the oxygen content of air, which is 21%. If, after a maximum time, the sensor signal does not reach a specified intermediate value, this is interpreted as a dead time error.
For future generations of vehicles, or model years, it is to be expected that a monitoring of the sensor dynamics will also be required when there is a decreasing oxygen concentration. Moreover, in hybrid vehicles in the future there will no longer be overrun phases, and thus no phases having a constant oxygen concentration of 21%.
Published German patent application document DE 10 2008 001 121 A1 describes a direction-dependent dead time monitoring. The patent discloses a method of diagnosing at least one exhaust gas sensor situated in the exhaust gas system of an internal combustion engine, in which a change in signal is compared with an expected change in signal. Here it is provided that a special operating state of the internal combustion engine is recognized, and that, in this special operating state of the internal combustion engine, for diagnosis a test injection is carried out that is torque-neutral or is not disturbing to the operator of the internal combustion engine. A special operating state can here be a overrun phase of the internal combustion engine. The determination of dead time TS can take place by determining an actual delay time from the time delay until the signal of the exhaust gas sensor follows the change in the exhaust gas composition, and comparing the actual delay time with a stored or calculated target delay time. A disadvantage here is that the active monitoring represents an intervention in the fuel system of the internal combustion engine, so that both fuel consumption and CO2 emissions are increased.
For the determination of dead times, outside engine controlling the principle of cross-correlation is known. This principle is described for example in “The Generalized Correlation Method for Estimation of Time Delay,” IEEE Transactions on Acoustics, Speech, and Signal Processing, vol. ASSP-24, no. 4, August 1976, by C. H. Knapp and G. C. Carter. A first and second signal to be compared, both triggered by the same cause, are then first filtered. Subsequently, the second signal is temporally delayed by a delay time. The signals obtained in this way are multiplied by one another corresponding to a cross-correlation function, and are integrated over a specified time span. A subsequently situated peak detector varies the delay time until a maximum of the cross-correlation function, i.e. of the multiplied and integrated signal, is present. At the maximum, the delay time corresponds to dead time T between the first and second signal. Dead time measurements using cross-correlation are used for example in laser distance measurement or in radar technology in order to determine the distance from an object.